1. Field of the Invention
This invention relates in general to output buffer circuitry, and relates in particular to such circuitry having improved response speed and noise characteristics.
2. Prior Art
There are many considerations in output buffer design, such as output loading, output switching speed, inductance of the bond-wire of the package, output capacitive loading, on-resistance of the output device, and power-supply noise due to output switching. A typical output buffer can be modeled by a RLC equivalent circuit shown in FIG. 1, where:
Vcc=positive power supply; Lcc=inductance of bond wire and package in upper branch; RI=on resistance of output pull up device; Ro=on resistance of output pull down device; Lss=inductance of bond wire & package in lower branch; Vss=ground; Vo=output voltage; and Co=output capacitive loading. PA1 (1) Optimize RI and Ro for given Co, Lcc, Lss and Q values for speed and noise. In most cases, speed will suffer. PA1 (2) Connect different input buffers to different power supplies so that output switching noise will not affect the input. PA1 (3) If there is only one power supply, then input buffer response time must be slowed down such that it will not be false triggered by noise. (4) Equalize output to mid-level before data is available such that the output swing is always half of the maximum.
Since Co, Lcc, Lss and the number of outputs Q on the power supply can not be arbitrarily changed, most prior art effort on improving output buffer design and performance has focused on one or more of the following approaches:
The noise involved in output buffers of the type involved here is primarily caused by feedback from the output to the input stage of the buffer. In cases where the low input to the buffer has a level of, say, 0.8 volts, this means that for any noise level of 0.8 volts or above the buffer will have difficulty in distinguishing between an input signal and noise. Similar problems can occur on the high level input to the buffer if the noise level tends to mask the signal.
The amount of noise generated is a function of the number of outputs associated with each power supply. In the ideal situation there would be one power supply for each output. However, from a practical standpoint this is not an attractive solution because of the costs and pin requirements involved.
The circuit of FIG. 1 can be analyzed as follows. Since the behaviors of the two half circuits are the same, only the half circuit including Vss needs to be considered Referring to FIG. 2, ##EQU1## From equations 1, 2 and 3: ##EQU2## From Equations 1, 3 and 4: ##EQU3## By dividing Equation 5 into Equation 6: ##EQU4## The solution to the above differential Equation (7) is: ##EQU5##
To understand the effects on the circuitry of variations in the values of different component in the above equations, the values of the output voltage V.sub.out(t) from prior art buffers vs. time are plotted and compared in the curves of FIGS. 3a, 3b, 4a, 4b, 5a and 5b. The curves are plotted for different numbers Q of output devices ranging from 1, 4, 8 and 16 output devices.
FIGS. 3a and 3b show the effects of variations in the value of Ro on the equations, with Lss=8.5nH and Co=35pF. Ro is plotted in two values, 10 ohms for FIG. 3a and 20 ohms for the FIG. 3b curves. The Ro=10 ohms curves of FIG. 3a indicate that the circuitry discharged to ground level faster than shown in FIG. 3b for a 20 ohm value; however, as shown by the vertical excursions of these curves, more noise was generated. As indicated above, the number of output devices Q on a given power supply is also a major factor in determining output speed and noise; as shown in FIGS. 3a and 3b, as the number of outputs increases (Q=1 to Q=16), the response speed decreases and the noise increases.
FIGS. 4a and 4b show the effects of variations of Lss on the equation, with Ro=20 Ohms and Co=35pF. Lss is plotted in two values, 8.5nH for FIG. 4a and 17nH for FIG. 4b. It can be seen that the 17nH curves of FIG. 4b show not only slower response time but also more noise than the 8.5.nH curves of FIG. 4a.
FIGS. 5a and 5b show the effects of variations in Co on the equation. With Ro=20 Ohms and Lss=8.5nH, Co is plotted in two values, 35pF for FIG. 5a and 75pF for FIG. 5b. It can be seen that the Co=75pF curves of FIG. 5b are slower but less noisy than the Co=35pF curves of FIG. 5a.